In a glass-producing apparatus, such as a vacuum degassing apparatus, the materials used for forming a vacuum degassing vessel serving as a channel for molten glass, and conduits for molten glass formed of a hollow tube, such as an uprising pipe and a downfalling pipe, are required to be excellent in heat resistance and corrosion resistance to molten glass. Although platinum or a platinum alloy, such as a platinum-rhodium alloy, is utilized as the materials satisfying such requirements in some cases (see Patent Document 1), refractory bricks, such as fused cast refractories, have been widely utilized because of being inexpensive in comparison with platinum (see Patent Document 2).
However, when refractory bricks, such as fused cast refractories, are utilized to form a conduit for molten glass, it is likely that components contained in the refractory bricks are eluted and mixed into the molten glass. Examples of the components include zirconia eluted from ziroconia-based fused cast refractories, and alumina eluted from alumina-silica-based fused cast refractories or ziroconia-based fused cast refractories.
If such components are uniformly dispersed in molten glass, it is unlikely that produced glass is adversely affected. However, unless such components are uniformly dispersed in molten glass, it is likely that produced glass has defects, such as reams.
On the other hand, when a conduit for molten glass is made of platinum or a platinum alloy, foreign substances are caused from such platinum or a platinum alloy in the interface between a platinum wall surface and molten glass in some cases. Foreign substances, which have been caused from such platinum or a platinum alloy and stay in molten glass, cause produced glass to have defects.
When a conduit for molten glass is made of refractory bricks, platinum or a platinum alloy, bubbles are formed in the interface between a conduit wall surface and molten glass in some cases. If bubbles are formed in the interface between a conduit wall surface and molten glass in the vacuum degassing vessel or a downfalling pipe (a conduit for molten glass downstream the vacuum degassing apparatus) of a vacuum degassing apparatus, the produced glass is caused to have defects since it is difficult to remove the bubbles from the molten glass.
The conduit structure for molten glass includes a system for removing foreign substances mixed in molten glass, such as a drain-out or an overflow. However, these conventional foreign-substance removing systems have failed to sufficiently remove components eluted from a refractory brick forming a conduit for molten glass, foreign substances caused in the interface between molten glass and a platinum wall surface forming a conduit for the molten glass, and bubbles formed in the interface between a conduit wall surface and the molten glass (hereinbelow, collectively referred to as “foreign substances etc. generated in the interface between molten glass and a platinum wall surface forming a conduit for the molten glass” or “foreign substances etc.”).
A flow of molten glass, which moves in a vacuum degassing vessel, contains unbroken bubbles or glass materials denatured by volatilization in some cases. If such substances are mixed in produced glass, it is necessary to remove such substances since the presence of such substances causes the produced glass to have defects. However, the conventional removing systems, such as a drain-out or an overflow has failed to sufficiently remove such unbroken bubbles or glass materials denatured by volatilization, which are contained in a flow of molten glass.
Patent Document 1: JP-A-2-221129
Patent Document 2: JP-A-11-139834